@Kabnine 

I have not seen TP-Link providing updated Deco firmware for X60 to choose how Satellite Deco will connect, but in your case there might be a workaround. It depends on what is Main Deco connected to: ISP modem or ISP gateway (modem/router). If you can give me ISP device brand name and model number, I can figure that for you and give details for workaround.

Other than that, the most powerful Deco in X-series is Deco X90, but even that might not be enough to work well it your setup. 

There is TP-Link Deco beta firmware for X55 that allows to choose where Satellite Deco connect, if you can replace your X60s with X55, that may fix the issue. See Deco X55 1.1.0

  @Alexandre. 

It sucks that the easiest fix is to buy more deco units that don't work properly unless using a beta firmware, but I found a 3 pack of the x55 for about half price used, so I've bought those.

Thank you very much for your help.

  @Kabnine 

Not every brand of WiFi mesh allows to configure how satellite nodes connect. That was the reason for initial pushback from TP-Link Deco development team: "Nobody does it, so why should we?"

If you ever decide to go with different brand of WiFi mesh, you now know to check for that feature before buying hardware.

Do you plan to sell X60s you have? If you are going to keep them, they can run side by side with X55s just fine. You can place X60s where you are OK with them to connect to Main Deco, and one of X60 units can be Main Deco.

  @Alexandre. 

I am just going to replace the shed and garage x60's with the x55's and tell them to connect to the master bedroom (it has Ethernet back haul). I am going to leave the 3 X60s (one is the main deco) in place since they have the 4 antenna (x55 is 2) and ethernet.

Not sure what I will do with the 2 x X60s I am replacing. Might try and flog them off on ebay or just give them to my brother as I get well and truly enough in house coverage from the 3 inside units.